The cotton boll weevil is easily one of the most notorious agricultural pests in the world. It occurs in all principal cotton-growing areas of Central and South America and the United States except parts of California. Wherever it is present, it is the key pest in cotton. The injury is caused by adults and the larvae or grubs. The adult weevils chew into or puncture the squares and bolls, and with their long slender bills, feed on the inner tissues. The eggs are laid in these holes, and the hatching grubs bore into the boll or square, causing the squares to drop off or wither and dry on the plant. This feeding either results in direct destruction of the flower or a reduction of fiber content in the boll. Losses can be so great as to be limiting. In 1982, damage to cotton in the United States was estimated at $429 million dollars. This figure has and is expected to continue to increase.
Chemical insecticides and cultural eontrois are currently employed in the control of boll weevil. These have associated problems and are not completely effective. There is a definite need for alternative materials that can be used in a complementary fashion with existing controls and to replace control agents that may lose efficacy due to resistance or other factors.
Management strategies for boll weevil control seek to attack the insect at the weakest point in its life cycle. Thus, early season adult control is based on insecticide treatment at pinhead square formation to kill overwintering adults prior to their damaging the current crop and beginning reproduction. Because insecticides kill benefieials and pests alike, the predator-prey balance is broken. Surviving insect pests proliferate due to the decreased populations of beneficial predatory insects, and additional insecticide applications become necessary for control of later pests, particularly the bollworm complex, Heliothis spp. Thus, removal of the early season boll weevil is tantamount for production of a viable lint crop. With increased emphasis on protection of the environment, reduction in the use of pesticides, and with increased urbanization and awareness, the need for biopestieides and insect management strategies is acute, and it should become an important factor in sustainable agricultural concepts now being rigorously supported by state and federal institutions. One of the most successful materials, Bacillus thuringiensis, has been tested extensively against a boll weevil but remains ineffective.
The sweet potato whitefly Bemisia tabaei (Gennadius) has appeared on poinsettias in California, Florida, Georgia and North Carolina. During 1981, the sweet potato whitefly was responsible for crop and market losses of 100 million dollars in cotton, eueurbits and lettuce in California and Arizona. The whitefly is increasingly a problem in Florida where, in 1986, this whitefly caused approximately 2 million dollars of damage to Florida's 8-10 million dollar poinsettia crop.
The sweet potato whitefly is also a pest of international importance, having been found on host plants throughout the mideast Caribbean and Central America. This insect is now known to feed on more than 500 different plants, many of which are of importance in the Caribbean and Florida. For example, cassava, sweet potato, squash, tomato, beans, lettuce, cotton, pepper, carrot, cucumber, eggplant, and water melon are all known hosts. This species of whitefly severely impacts infested plants by its feeding, production of honeydew with resultant growth of sooty mold, and transmission of plant pathogens. Most extensive losses to this pest have been through direct feeding damage and indirect damage through transmission of plant diseases.
Whitefly-borne diseases are of major importance in tropical and subtropical agriculture. More than 70 diseases caused by viruses and microorganisms are known to be transmitted by whiteflies, with most of them being transmitted by the sweet potato whitefly. In Puerto Rico, this whitefly is a vector of at least seven diseases. One of these diseases is the bean golden mosaic virus, a disease affecting many legumes.
The sweet potato whitefly has proven to be very difficult to control with conventional pesticide applications. Many factors contribute to the lack of control obtained with pesticides. The most important factor is that this whitefly has demonstrated a broad spectrum of resistance to chlorinated hydrocarbons, organophosphorus, carbamate, and synthetic pyrethroid insecticides. Very few commercially available pesticides are effective against whiteflies, and those which do work are only effective if care is taken to make a very thorough application of the insecticide several times a week. The sweet potato whitefly spends most of its life on the undersides of leaves, therefore, growers must adjust their management practice to permit increased pesticide coverage there. The spacing of the plants must be such that the chemical spray can penetrate the canopy and reach all surfaces of the plants.
In addition to being largely ineffective, and difficult and costly to apply, chemical control of these pests has other significant drawbacks. For example, the use of chemical pesticides presents the further disadvantages of polluting the environment, creating potential health hazards to agricultural workers and to consumers, development of resistance to chemicals in target pest species, detrimental effect of these chemicals on non-target species resulting in secondary pest outbreaks, and phytotoxic reaction by treated plants.
Because of the problems associated with the use of chemical pesticides, safer and more effective methods of control for insect pests are clearly needed. Although biological control agents are actively being sought after, to date no biological control agent has been commercially successful for the control of the boll weevil and the sweet potato whitefly.
Biological control agents have been tried; however, availability, limited host range, cost and reliability have reduced the potential for implementing the use of these biological control agents. The development of a broad spectrum of pesticides would reduce the need for many of the petrochemically based pesticides. By using fungi to control insect pests, the potential for resistance development is minimized, which, in turn, will stabilize many of the pest management programs.
In many instances, fungi used to control insect pests have not had the effectiveness required for commercial use. Therefore, there also exists a need for enhancing the effectiveness of such entomopathogenic fungi.